Regulation of neural stem cell behaviors in Drosophila

Abstract

The ability of an organism to develop, maintain homeostasis and recover from injury is critically dependent on the behavior of resident stem cell populations within its tissues. In particular, neural development involves highly orchestrated patterns of stem cell division followed by loss of stem cell proliferation. This pattern of gradual restriction of neurogenesis is seen in both mammals and invertebrates such as the fruit fly, Drosophila melanogaster. Multiple fate decisions have been described for neural stem cells in Drosophila, including apoptosis, quiescence and terminal division. The terminal fates of mammalian neural stem cells remain unclear, due to a lack of precise genetic and molecular tools to identify, track and manipulate cell fates in this system. The developing Drosophila nervous system thus offers the most complex tissue available to study neural stem cell behaviors in vivo. This thesis describes the regulation of neural stem cell behaviors in the developing Drosophila embryo. I investigated patterns of proliferation and programmed cell death in this system and found the apoptotic death of abdominal embryonic neural stem cells is cell cycle independent. I also described a novel form of G2 quiescence in post-embryonic neural stem cells, as well as the identification of a novel terminal fate for embryonic neural stem cells. Next, I investigated the role of the transcription factor Dichaete, a homologue of Sox2, in regulating pro-apoptotic gene transcription in neural stem cells. In this study, we found that Dichaete is necessary and sufficient to prevent activation of cell death in neural stem cells and other tissues in the developing embryo. I investigated the mechanism of transcriptional regulation of apoptosis by assessing long-range chromatin interactions within the cell death locus in Drosophila larval central nervous system tissue. Finally, I generated epitope-tagged alleles of the cell death genes grim and reaper, which allow us to visualize protein expression of these pro-apoptotic factors for the first time in vivo. The results from this thesis have furthered our knowledge of potential neural stem cell behaviors and deepened our understanding of the regulation of programmed cell death in this population.